Does anyone have feedback on how to implement this type of dependency correctly in a Guix service?
On Sun, Jun 7, 2020 at 11:43 AM conjaroy <[email protected]> wrote: > Greetings help-guix, > > I've been a casual user of Nix for a couple of years and have decided to > test the waters with Guix. While I'm looking forward to spending time with > Lisp after many years away, my biggest impression is that Guix seems to > have well-documented interfaces in cases where Nix relies more on loose > conventions. > > After reviewing the manual and some of the service definitions, I'd like a > better understanding of how to implement a common pattern. Let's say that I > have some application Foo that uses an external system for persistence, > like a SQL database. Before starting up service Foo I need to ensure both > that the database service is running and that the database instance for Foo > has been initialized, because Foo doesn't know how to initialize the > database on its own. > > The first issue (how to ensure that the database service is up) seems to > be solved by adding a shepherd-root-service-type service extension that > declares a set of "requirements". And the second issue (performing > pre-startup initialization) seems to be handled by the > activation-service-type extension. So far so good. > > But I couldn't find documentation on whether service activation scripts > can safely rely on other services that happen to be declared as > requirements in the shepherd-root-service-type extension. And while I found > many activation scripts that do simple things like modifying the > filesystem, I couldn't see any that interact directly with other services. > However, I did see some evidence of service extensions relying on the side > effects of other service extensions: a number of activation scripts call > "getpwnam" for info on system accounts that could exist only if the > corresponding account-service-type extension has already been executed. > > So my questions are: could someone clarify best practices for initializing > state in Service A before Service B starts up? And is there anything about > the ordering/dependencies of a service's extensions that could be better > documented in the manual? > > Thanks for all of your work on this project. > > Jason >
